29 p. : 26 cm

"Comparative anatomical studies have been made of the families Scissurellidae, Pleurotomariidae, and Fissurellidae. Shared primitive characters such as paired organs and shared derived characters like epipodial tentacles, indicate that the Scissurellidae should be removed from the Pleurotomariacea, despite nearly identical external morphology, and placed in the Fissurellacea. The only available pleurotomariaceans when the scissurellids appeared in early Cenozoic time are the haliotids and the Pleurotomariidae. Neither family has as many shared characters as the scissurellids and the fissurellids. A study of the ultrastructure of the scissurellids further justifies this decision. Ultrastructure normally is quite conservative in major taxa. The pleurotomarians have a complex prismatic-nacreous wall, whereas the scissurellids have a simple prismatic-'type-2 crossed-lamellar' wall. This latter wall type is found in the embryonic stage only of the fissurellids; in adults the wall is crossed-lamellar-crossed-lamellar. The early embryonic stages of the fissurellids are similar or nearly identical with all growth stages in the scissurellids. This observation suggests that the scissurellids might have been neotenously derived from the fissurellids. The scissurellids are remarkably convergent on several genera of the Upper Paleozoic Eotomariidae and the Triassic Temnotropidae in external morphology"--P. [1]

Title from caption

"February 20, 1975."

Includes bibliographical references (p. 28-29)

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